The present invention relates to the field of plastics and more particularly, to a general method for the preparation of plastic objects, plastics, polyester/polyamide resins and polymers from amino acids, dipeptides, tripeptides, polypeptides, proteins and/or other proteinaceous material. The products obtained by the process of the
Due to environmental considerations, the disposal of solid waste is a major industrial concern. One of the most difficult to dispose types of waste is non-biodegradable plastics used, for example, in agricultural applications and packaging. Concerns over the effect of non-biodegradable plastics have resulted in a worldwide prohibition of plastic disposal at sea, increasingly severe regulatory limits on use of non-biodegradable plastics and incentives to reduce the amounts of non-biodegradable plastics used. These trends have resulted in an increased demand for biodegradable plastics, especially in the fields where recycling is difficult such as packaging materials and agricultural plastics (e.g. mulch films).
The biodegradable plastics known in the art have been accepted only sparingly, primarily due to the high cost of the final product, the high cost primarily being determined by the high cost of sufficiently pure raw materials.
A further disadvantage of biodegradable plastics known in the art is the rather limited range of properties that are available. Obviously, different implementations require plastics with different physical properties such as barrier properties, glass transition temperature, viscosity, stickiness, pliability, transparence/opaqueness, malleability, tear-resistance and the like.
Currently available commercially important biodegradable plastics fall into three main categories: polylactides, starch-based polymers and polyhydroxyalkanoates.
Polylactide polymers (PLA) are produced by polymerization of lactic acid. Lactic acid is first oligomerized with the loss of water to yield linear chains of polylactic acid. The polylactic acid is de-polymerized to yield lactide, a cyclic dimer The six-member ring of the lactide is purified and subjected to ring-opening polymerization to produce PLA resin. Cargill Dow Polymers LLC (Minneapolis, Minn., USA) is the largest producer of PLA resins. PLAs can be processed by most melt fabrication techniques including thermoforming, sheet and film extrusion, blown film processing, fiber spinning and injection molding. Applications include coatings for paper plates. A disadvantage of PLA plastics is that the linearity of the molecules limits the range of physical properties that such plastics can have. This is most critical as such polymers generally have a relatively high glass transition temperature, Tg. Further, as the polymers are linear, every hydroxy or carboxylic impurity present during polymerization results in chain termination leading to low molecular weight products with poor chemical and mechanical stability. Related to PLA is PGA, a polyester plastic derived from polyglycolic acid.
The main starch-based plastics are starch/ethylene-vinyl alcohol copolymer blends. These blends are prepared by blending a starch-based component and an ethylene-vinyl copolymer in an extruder in the presence of water or other plasticizer. The temperature and pressure conditions destructurize the starch and the resulting extruded composition forms a thermoplastic melt. Such plastics have physical properties similar to polystyrene or polyethylene but are moisture-sensitive. Novamont SpA (Novara, Italy) produces such resins containing 60% starch that are marketed under the name Mater-Bi® and can be shaped by film blowing, extrusion, injection molding or thermoforming. Novon International (Tonawanda, N.Y., USA) produces a resin containing up to 95% starch and marketed under the name Novon®. It is important to note that such materials are not truly biodegradable as the non-starch component of the plastic remains as minute particles.
Polyhydroxyalkanoates (PHA) plastics are linear polyesters produced by bacterial fermentation of sugar or lipids. The production of PHA plastics is described in Doi. Y. Microbial Polyesters, VCH Publishers, New York, N.Y., 1990. More than 100 different monomers can be combined within this family to give materials with a wide-range of properties. PHAs can be either thermoplastic or elastomeric materials, with melting-points ranging from 40° to 180° C. and are suitable for blow molding, injection molding and extrusion. The leading producer of PHAs is the Monsanto Company (St. Louis, Mo., USA) manufacturing a polyhydroxybutyrate under the name Biopol®. PHAs are entirely biodegradable but very expensive.
In the art, proteins have been used as a primary component in the production of plastics. Pure protein plastics are generally fragile and absorb moisture. To improve the mechanical and barrier properties of protein-based plastics, the protein component is generally modified to increase hydrophobicity.
In 1899, Adolf Spitteler received a German patent (Encyclopedia of Polymer Science and Technology, Interscience Publishing, NY, 1965, vol. II, p. 695.) for casein formaldehyde first sold under the name Galalith. Casein formaldehyde is a thermoset plastic made by steeping solid casein workpieces in formaldehyde. Casein formaldehyde is still used in the manufacture of buttons.
Ghorpade et al. (Ghorpade, V. M.; Lin, H.; Hanna, M. A. “Physical and mechanical properties of protein-polyethylene extrudate” ASAE Paper No. 93-6533, 1993) teach the use of chemicals such as formaldehyde and dimethoxy dimethyl silane to increase the hydrophobicity of the protein used as a coextrudate with polyethylene to make a composite plastic.
U.S. Pat. Nos. 5,397,834, 5,523,293 and 5,710,190 teach the combination of pure soy protein, a foaming agent, a plasticizer and water to make a biodegradable thermoplastic material. Such plastics comprise soy protein cross linked with an aldehyde starch. The high cost of sufficiently pure soy protein makes such plastics uncompetitive with prior art polyethylene.
There is a widely recognized need for, and it would be highly advantageous to have a general method for producing plastics from inexpensive raw materials. There is also a widely recognized need for, and it would be highly advantageous to have a general method for producing biodegradable plastics. There is also a widely recognized need for and it would be highly advantageous to have a general method for producing biodegradable plastics having many and varied physical properties.